PROJECT SUMMARY/ABSTRACT Deep tissue abscesses remain a common cause of morbidity, mortality, and hospital stay despite development of image-guided percutaneous drainage accompanied by antimicrobial treatment. Abscess aspirates have been shown to contain multiple ESKAPE species, a group of antibiotic-resistant bacteria. Photodynamic therapy (PDT), which exerts antimicrobial activity through generation of reactive oxygen species, is efficacious against antibiotic-resistant strains and does not lead to acquired resistance. PDT could complement the current standard of care by sterilizing the abscess cavity following drainage, thereby potentially replacing antibiotics. A Phase 1 clinical trial to establish the safety and feasibility of methylene blue (MB) PDT of abscess cavities is ongoing (ClinicalTrails.gov identifier: NCT02240498). The efficacy of this treatment is dependent on optical fluence rate, fluence, and photosensitizer concentration. However, optical absorption and scattering of the abscess wall and microbial uptake of MB within human abscess cavities have not been quantified. This lack of optical property data precludes pre-treatment planning for abscess PDT, which reduces treatment efficacy and increases the risk of high fluence rates that would cause thermal damage. The objective of this study is to rigorously plan the delivered light dose during PDT of abscess cavities by measuring abscess wall optical properties and MB uptake in human subjects, using an optical spectroscopy system. Patient-specific measurements will allow for design of treatment plans that achieve the target light dose in a larger proportion of the abscess wall, and minimize optical power and risk of thermal damage. This will lay the foundation for a rigorous Phase 2 clinical trial, with the ultimate goal of reducing abscess recurrence and post-procedure intervention, and decreasing or eliminating reliance on antibiotics. Aim 1. Develop and validate an optical spectroscopy system for assessment of optical properties at the boundary of a hollow cavity. A fiber bundle-based optical probe will be designed and fabricated. This probe will be used to perform spatially-resolved diffuse reflectance and fluorescence spectroscopy to extract optical properties and MB concentration. Accuracy will be assessed in tissue-simulating phantoms. Aim 2. Measure abscess wall optical properties and methylene blue uptake in humans. Optical measurements of abscess walls will be made in 15 subjects participating in the described Phase 1 clinical trial. These measurements will be made prior to and following MB instillation and aspiration. Aim 3. Optimize treatment plans for clinical PDT of deep tissue abscesses. Measured optical properties, along with subject CT images, will be used to generate retrospective treatment plans for each participating subject. These treatment plans will be compared to the fixed light dose and scatterer concentration delivered in the parent Phase 1 trial.